Coupling device and instrument for connecting the coupling device to a head of a bone anchor

ABSTRACT

A coupling device includes a receiving part having a head receiving portion for pivotably receiving a head of a bone anchor and a rod receiving portion defining a recess for receiving the rod, the rod receiving portion having an engagement structure for engaging a locking member to lock the rod in the recess and a first engagement surface different from the engagement structure for engaging an instrument, wherein the first engagement surface is formed away from radially outwardly facing regions of the rod receiving portion, and a locking ring positionable around the head receiving portion and having a second engagement surface for engaging the instrument. The locking ring can assume an insertion position where the head of the bone anchor is insertable into the head receiving portion, and a pre-locking position where the head is prevented from removal from the head receiving portion.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No.17/123,959, filed Dec. 16, 2020, which claims priority to and thebenefit of U.S. Provisional Patent Application No. 62/949,872, filedDec. 18, 2019, the contents of which are hereby incorporated byreference in their entirety, and claims priority from European PatentApplication EP 19 217 559.4, filed Dec. 18, 2019, the contents of whichare hereby incorporated by reference in their entirety.

BACKGROUND Field

The application relates to a coupling device for coupling a rod to abone anchor and to an instrument for connecting the coupling device to ahead of the bone anchor. More specifically, the invention relates to acoupling device and to an instrument which permits placement of areceiving part with an outer locking ring onto the head in situ, i.e.,in a condition in which the bone anchor has already been inserted into abone or a vertebra with the head protruding out of the bone surface.

Description of Related Art

A coupling device with an outer locking ring that is arranged around ahead receiving portion is known, for example, from US 2018/0055545 A1.The coupling device is part of a polyaxial bone anchoring device thatincludes a receiving part with a rod receiving portion and a headreceiving portion and a locking ring arranged around the head receivingportion which forms a compression member to exert pressure onto aninserted head. The head receiving portion has a plurality of slits thatrender the head receiving portion flexible so that when pressure isexerted onto the head receiving portion by the locking ring, the headcan be clamped and finally locked. An instrument for moving the lockingring from an insertion position for the head to a locking position forthe head and vice versa is also described in the document. Usually, thebone anchoring device is pre-assembled with the bone anchor.

U.S. Pat. No. 10,258,390 B2 describes a device for placing a receivingpart of a bone anchoring device onto a head of a bone anchoring element,which includes a holding portion, a collet configured to be held in theholding portion, the collet having a tubular body and a distal endportion configured to be flexibly expanded and compressed in a radialdirection, and a plunger configured to be positioned in the collet. Withthe device it is possible to engage the receiving part at an inner walland to place it onto the bone anchor that has been already inserted intoa bone. The bone anchoring device described in the document is of thetype using an inner cap placed in the receiving part as a compressionmember for exerting pressure onto the head with a rod or a lockingmember to lock the head.

SUMMARY

While bone anchoring devices using an inner cap for clamping and lockingthe head described in the prior art may be appropriate for a number ofclinical applications, there is still a need for an alternative and/orimproved bone anchoring device which allows in situ placement of acoupling device including a receiving part and a compression member ontoan implanted bone anchor.

It is therefore an object of the present invention to provide a couplingdevice and an instrument for use with such a coupling device that isimproved compared to known devices.

A coupling device according to embodiments of the invention may includean outer locking ring as a compression member to exert pressure onto thehead, and can be placed onto a bone anchor inserted in bone or in avertebra in situ. Such in situ placement requires proceeding carefullyso as to avoid injuries by exerting forces that are too strong. Theinstrument provides a hand-held device that permits easier placement ofthe coupling device in situ. This can be achieved gently withoutexerting excessive forces that may cause injuries. In particular, inspinal surgery, such a procedure may be advantageous.

Moreover, the coupling device permits, once placed onto the head of thebone anchor, a temporary locking without the requirement of a rod or alocking member. The rod can even be inserted and is not required to siton the rod support but can be at an elevated position. Hence, by movingthe locking ring from a locking position to a pre-locking position andvice versa, the bone anchoring device can be locked and unlocked topermit various adjustment steps. As such, the rod and/or the lockingmember are only truly needed for the final locking at the end of thesurgical procedure. Therefore, the in situ placement of the couplingdevice with the capability of temporary and/or variable locking of thehead increases the variety of correction steps that can be carried outduring surgery.

With the instrument, the locking ring can be moved into the pre-lockingposition after placement onto the head of the bone anchor. This preventsremoval of the coupling device from the head of an inserted bone anchoronce the coupling device has been connected to the head of the boneanchor. Thus, the instrument can be safely detached.

According to an embodiment, the coupling device is configured to beengaged by the instrument at an inner wall of legs of the couplingdevice formed by the rod channel. Since an outer surface of thereceiving part is not engaged by the instrument, the necessary space forplacing the coupling device onto the head of a bone anchor alreadyinserted into bone may be reduced. This permits use of narrower channelsthrough the tissue of the human body to approach the implantation site.Hence, the instrument and the coupling device may be particularlyapplicable in minimally invasive surgery (MIS).

Moreover, according to an embodiment, the arms of the instrument areresiliently flexible, which gives a tactile response when attaching theinstrument to the coupling device. This facilitates easier and moresecure attachment of the instrument to the coupling device.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will become apparentfrom the description of embodiments by means of the accompanyingdrawings. In the drawings:

FIG. 1 shows a perspective view of a portion of a spinal column withimplanted bone anchors and an instrument attached to a coupling deviceaccording to an embodiment of the invention.

FIG. 2 shows an enlarged portion of FIG. 1.

FIG. 3 shows a cross-sectional view of the embodiment of the couplingdevice and the instrument in FIGS. 1 and 2 placed over a head of a boneanchor, wherein the cross-section is taken in a plane including acentral longitudinal axis of the instrument and that extends throughcenters of legs of the receiving part.

FIG. 4 shows a perspective view from a top of a receiving part of thecoupling device of FIGS. 1 to 3.

FIG. 5 shows a perspective view from a bottom of the receiving part ofFIG. 4.

FIG. 6 shows a top view of the receiving part of FIGS. 4 and 5.

FIG. 7 shows a cross-sectional view of the receiving part of FIGS. 4 to6, the cross-section taken along line A-A in FIG. 6.

FIG. 8 shows a perspective view from a top of a locking ring of thecoupling device of FIGS. 1 to 3.

FIG. 9 shows a perspective view from a bottom of the locking ring ofFIG. 8.

FIG. 10 shows a top view of the locking ring of FIGS. 8 and 9.

FIG. 11 shows a cross-sectional view of the locking ring of FIGS. 8 to10, the cross-section taken along line B-B in FIG. 10.

FIG. 12 shows a perspective view of a front portion of the instrument ofFIGS. 1 to 3.

FIG. 13 shows a cross-sectional view of the front portion of theinstrument of FIG. 12, the cross-section taken in a plane including acentral longitudinal axis of the instrument and extending throughcenters of arms of the instrument.

FIG. 14 shows a perspective view from a top of an end portion of aholding member of the instrument of FIGS. 12 and 13.

FIG. 15 shows a perspective view from a bottom of the holding member ofFIG. 14.

FIG. 16 shows a side view of the holding member of FIGS. 14 and 15.

FIG. 17 shows a cross-sectional view of a front portion of the holdingmember of FIGS. 14 to 16, the cross-section taken in a plane includingthe central longitudinal axis of the instrument and extending throughcenters of arms of the holding member.

FIG. 18 shows a perspective view from a top of a front portion of apushing member of the instrument of FIGS. 12 to 13.

FIG. 19 shows a perspective view from a bottom of the pushing member ofFIG. 18.

FIG. 20 shows a side view of the front portion of the pushing member ofFIGS. 18 and 19.

FIGS. 21a to 21d show side views of steps of attaching the instrument tothe coupling device according to an embodiment of the invention.

FIG. 22 shows a perspective view of a step of fixing the front portionof the instrument to the coupling device.

FIG. 23 shows a perspective view of a step of adjusting a pre-lockingposition of the locking ring relative to the receiving part with theinstrument.

FIGS. 24a to 24c show steps of using the instrument to place thecoupling device onto a head of a bone anchor inserted into bone.

FIG. 25 shows a perspective view of a portion of a spine with insertedbone anchors and coupling devices, and attachment of another instrumentfor locking the coupling devices relative to the bone anchors.

DETAILED DESCRIPTION

FIGS. 1 to 3 illustrate a first embodiment of a coupling device 10engaged by an instrument 100 for placing the coupling device onto a headof a bone anchor. According to an embodiment, the bone anchor 1 canalready be inserted into bone, for example, a pedicle of a vertebra 500,prior to mounting the coupling device thereon. The bone anchor 1includes a shank 2 configured to be anchored in bone and a head 3. Theshank 2 may have a bone anchoring structure on at least a portionthereof, such as a bone thread or barbs or any other suitable structurefor anchoring. The head 3 may have a spherical outer surface portionthat includes a largest outer diameter of the sphere. At a free end ofthe head 3, an engagement recess 4 may be provided for engagement with ascrew driver. The coupling device 10 is configured to couple the boneanchor 1 to a rod (not shown), which is configured to be inserted intoand fixed in the coupling device. As can be seen in particular in FIG.3, the coupling device 10 includes a receiving part 20 and a lockingring 40. The receiving part serves for accommodating the head 3 and forreceiving the rod (not shown). The locking ring 40 is configured toclamp and/or lock the head 3 in the receiving part 20. The bone anchor 1and the coupling device 10 form a polyaxial bone anchoring device thatis configured to permit coupling of the rod to the bone anchor based onvarious angular positions of the coupling device.

Referring additionally to FIGS. 4 to 7, the receiving part has a firstor upper end 2 a and a second or lower end 2 b. A longitudinal centralaxis C extends from the upper end 2 a to the lower end 2 b. Adjacent tothe upper end 2 a, the receiving part has a rod receiving portion 21,and adjacent to the lower end 2 b, the receiving part has a headreceiving portion 22. The rod receiving portion 21 is substantiallycylindrical, extends along the central axis C, and has a passage 23 thatextends from the upper end 2 a into the head receiving portion 22. Thepassage 23 may have regions with different diameters and/or shapes. Inthe embodiment, a first region 23 a may be a coaxial bore with a firstdiameter that extends from the top end 2 a to a distance from the headreceiving portion 22. The first region 23 a may be followed by a secondregion 23 b which is narrower, which in turn may be followed by a thirdregion 23 c with an even more reduced diameter and which opens into thehead receiving portion 22. Between the first region 23 a and the secondregion 23 b, a step 23 d may be provided. Similarly, between the secondregion 23 b and the third region 23 c, a step 23 e may be provided. Inaddition, in the second section 23 b, two opposite axially extendingcylinder segment-shaped recesses 39 may be formed, as best seen in FIG.4. The recesses 39 serve as a guiding structure for a portion of theinstrument 100. It shall be noted that the detailed shape of the passagemay be different from the embodiment shown.

A substantially U-shaped recess 24 with a bottom 24 a extends from thetop end 2 a to a distance from the head receiving portion 22. By meansof the substantially U-shaped recess 24, two free legs 25 are formed.The recess 24 provides a channel for receiving the rod. An internalthread 26 is provided on the legs 25 and extends from substantially theupper end 2 a to a distance from the bottom 24 a of the recess 24. Inother words, the internal thread 26 is provided along substantially thefirst region 23 a of the passage. The internal thread 26 may be, forexample, a square thread which may be advantageous for preventingsplaying of the legs 25. At the lower end of the internal thread 26 thatfaces towards the head receiving portion 22, an undercut 26 a may beformed. At a distance from the upper end 2 a, a groove or otherwiseweakened section 27 is provided that has a reduced wall thickness andallows breaking off of the upper portion of the legs 25. At an edge ofthe weakened section 27 in a circumferential direction,circumferentially extending recesses or cutouts 27 a may be formed thatprovide further weakening of the weakened section 27 and may facilitateeasier breaking off of the upper portion of the legs 25. Also, insidethe passage 23, the internal thread 26 may be interrupted at an axialposition that corresponds to a lower edge of the weakened section 27.Hence, the legs above the weakened portion 27 can serve as extendedtabs. With the extended tabs, it is possible to manipulate the polyaxialbone anchoring device with an inserted rod that may be at a higherposition with respect to the bottom 24 a of the recess 24. By means ofthis, for example, a vertebra can be pulled against the rod.

At an outer surface of the rod receiving portion 21, an engagementstructure for engagement with an instrument, for example, an instrument300 as shown in FIG. 25, may be provided. The engagement structure mayinclude circumferentially extending ribs 28, which may be arrangedasymmetrical with respect to a plane including the central axis C of thereceiving part 20 and a channel axis L of the substantially U-shapedrecess 24. That means, a first rib can start at a distance from the edgeof the U-shaped recess 24 on one side and extend to a distance aroundthe receiving part 20. A second rib can start at the opposite siderelative to the U-shaped recess and extend to a distance around the rodreceiving portion. Thereby, a rib-free surface 29 is formed on each sideof the U-shaped recess 24. Below each of the ribs 28, a substantiallyflat outer surface portion 30 may be formed that may have a contour of arectangle or square and may serve for engagement with a portion of thelocking ring 40. Between the ribs 28 and the substantially flat surfaceportion 30, two latching grooves or steps 31 a, 31 b may be provided oneafter the other in an axial direction. The latching grooves 31 a, 31 bare configured to cooperate with a portion of the locking ring 40 toprovisionally hold the locking ring at one or more specific positions.The upper latching groove 31 a may serve for preventing further upwardmovement of the locking ring in an insertion position. The lowerlatching groove 31 b may serve for latching the locking ring in apre-locking position.

At a position between the weakened section 27 and the bottom 24 a of thesubstantially U-shaped recess 24, an engagement structure 32 forengagement with the instrument 100 may be provided. The engagementstructure 32 may be formed as a shallow rounded groove, preferablysubstantially cylindrically-shaped, and may be located at the edge ofthe legs 25. Four such grooves may be provided, two on each leg 25 withone on either side of the leg 25. An axial position of the engagementstructure 32 may be slightly below the weakened portion 27. Preferably,the shape of the engagement structure 32 is such that a correspondingengagement portion of the instrument 100 which is adapted thereto canslide into the engagement structure 32 and can slide out of theengagement structure 32. More specifically, the engagement portion ofthe instrument can resiliently snap into the engagement structure 32.

Furthermore, cutouts 33 may be provided on either side of thesubstantially U-shaped recess 24, which may serve for receivingprojections of the locking ring 40 therein. By means of this, thelocking ring 40 can be secured against rotation relative to thereceiving part 20. From a position above the cutouts 33 to the headreceiving portion 22, an external diameter or width reduction of the rodreceiving portion 21 forms a step 34 that serves as a further abutmentfor the locking ring.

The head receiving portion 22 has a substantially cap-like shape with ahollow substantially spherical interior portion 35 forming a seat forreceiving the head 3 pivotably therein. A plurality of slits 36 a, 36 brender the head receiving portion 22 flexible, so that, when the head 3is inserted into the hollow interior 35, the head receiving portion 22expands. When pressure is exerted onto an inserted head 3 by the lockingring 40, the head receiving portion 22 is compressed. The slits are opentowards the lower end 2 b and extend along the spherical section. Slits36 a end at the end of the spherical section, while slits 36 b mayextend closer to or even into a region of the rod receiving portion thatdefines the third region 23 c of the passage 23. An outer surface of thehead receiving portion 22 may be recessed in a radial direction relativeto a lower end of the rod receiving portion 21. The outer surface mayhave a bottom section 37 a that may be cylindrical or slightly outwardlytapered. The bottom section 37 a forms an annular projection that isconfigured to cooperate with a corresponding portion of the locking ring40. The bottom section 37 a is followed by a circumferential groove andby a radially outwardly bulged portion 37 b, which is configured tocooperate with another portion of the locking ring 40. The bulgedportion 37 b narrows in the direction towards the rod receiving portion,which contributes to the recessed shape of the head receiving portion22.

Turning now to FIGS. 8 to 11, the locking ring 40 will be described. Thelocking ring 40 is designed to encompass the head receiving portion 22and has an internal surface structure that facilitates, in cooperationwith the head receiving portion 22, a full locking of an inserted head 3in the head receiving portion 22 when the locking ring is at itslowermost position. It further facilitates a pre-locking when thelocking ring is at a position slightly above the lowermost position,which still allows pivoting of the head 3 in the head receiving portion22, but prevents removal of the head 3 from the head receiving portion22. Lastly, the locking ring is configured to permit insertion of thehead 3 into the hollow interior 35 when the locking ring is at anuppermost position relative to the receiving part.

In greater detail, the locking ring 40 has a lower surface 4 b and anopposite upper surface 4 a that is substantially ring-shaped. Adjacentto the lower surface 4 b, there is a first radial projection 41 whichprojects inwardly and is configured to cooperate with the outer portion37 a at the head receiving portion. The annular projection 41 isfollowed by a widened section 42, which is then followed towards theupper surface 4 a by a second annular projection 43 that also projectsinwardly. As depicted in FIG. 3, when the locking ring 40 is mountedaround the head receiving portion 22, the first annular projection 41 isconfigured to press onto the first portion 37 a of the head receivingportion and the second annular projection 43 is configured to press ontothe bulged portion 37 b.

From the upper surface 4 a of the ring-shaped portion of the lockingring 40, two projections 44 protrude upwardly and have a free endsurface forming a rod support surface 44 a for a stabilization rod. Theprojections 44 are diametrically opposite, i.e., offset by 180°, fromone another. The rod support surface may have a substantially V-shapedcross-section to permit safe support of rods of different diameters.When the locking ring 40 is mounted to the receiving part 20 such thatthe upper surface 4 a faces towards the upper end 2 a of the receivingpart, the projections 44 may extend through the cutouts 33 and therebysecure the rotational orientation of the locking ring 40 relative to thereceiving part 20. The locking ring 40 also includes two upstanding arms45 that are positioned asymmetrically with respect to a plane thatextends through the central axis C and through the centers of the rodsupport surfaces 44 a, for example, in the same or similar manner as theribs 28 of the rod receiving portion 21 are arranged on the receivingpart 20. At an upper end of the arms 45, an engagement portion in theform of circumferential ribs 46 defining a groove 46 a are provided forengagement with an instrument. Such an instrument can be, for example,an instrument 300 as shown in FIG. 25. An inwardly facing upper edge 45a of the arms 45 may be provided that is configured to engage thelatching grooves 31 a, 31 b at the receiving part 20. In the assembledstate, the engagement structure in the form of the ribs 46 with thegroove 46 a is aligned with the engagement structure 28 at the receivingpart 20, leaving the rib-free surface 29 of the receiving part 20exposed. Moreover, the arms 45 have a substantially flat inner wall thatis configured to engage the flat portion 30 at the rod receiving portion21. Thereby, a further form-fit connection is established between thelocking ring 40 and the receiving part 20. Between the projections 44and the arms 45, there are upstanding flexible portions 47 that have aheight slightly greater than that of the projections 44 and which haveat an inner wall thereof a ledge 47 a which is configured to abutagainst the step 34 on the receiving part 20.

A bone anchoring device according to an embodiment may further include arod that is configured to be inserted into the substantially U-shapedrecess 24 and a locking member, such as a set screw, that is configuredto be screwed between the legs 25.

Referring to FIGS. 12 to 20, an embodiment of the instrument 100 forplacing the coupling device 10 onto a head 3 of a bone anchor 1 will bedescribed in greater detail. As shown additionally in FIG. 1, theinstrument 100 includes a substantially tubular holding member 101 and ashaft-like pushing member 120 that is arranged inside the holding member101. The holding member 101 has a rear portion that is connected to orprovided with a handle 102 and an opposite front portion. The frontportion is bifurcated through a slot 104 that forms two arms 105 whichare configured to be flexibly compressible and expandable in atransverse direction relative to a longitudinal tube axis T of theholding portion. The slot 104 extends from a free end portion 101 a ofthe holding member 101 in a substantially U-shaped manner with a bottomfacing towards a rear end of the holding member. From the bottom of theslot a substantially straight portion 104 b continues and opens into anelongate portion 104 c. It shall be noted that the slot may havedifferent shapes for obtaining various different degrees of flexibility.

The arms 105 have a front portion 105 a adjacent to the free end portion101 a. The front portion 105 a is recessed from an overall cylindricalshape of the holding member so as to fit into the rod receiving portionof the receiving part 20. Through the recessed arrangement of the frontportion 105 a of the arms 105, an abutment face 106 is provided at theend of the front portion 105 a which may abut against the upper end 2 aof the receiving part when the front portion 105 a of the arms isinserted into the rod receiving portion. The front portions 105 a of thearms each has a recess 107 adjacent to the free end 101 a, which may besubstantially rectangular. By the recess 107, two short legs 107 a areformed on each of the front portions 105 a of the arms 105. This designpermits the front portions 105 a of the arms to extend downward into alowermost region of the substantially U-shaped recess 24. An outersurface of the front portion 105 a includes an axially extendingsubstantially cylinder segment-shaped rib or projection 108 which isconfigured to engage the cylindrical recess 39 in the second section 23b of the rod receiving portion 21.

Furthermore, at a distance from the abutment face 106, each arm 105includes an engagement portion 109 which is configured to engage theengagement structure 32 at the rod receiving portion 21. In greaterdetail, the engagement portion 109 may be formed as a cylindricalprojection with a cylinder axis transverse to the longitudinal axis T ofthe holding portion and arranged and configured to engage the groove 32.As can be seen in particular in FIGS. 12 and 14 to 15, each frontportion 105 includes two engagement portions 109 which are located tothe left and to the right of the axial cylindrical projection 108. Thearms 105 may be substantially flat at their outer surface except in theregion of the axial projection 108 and the engagement portions 109. Atan inner wall of the front portions 105 a, coaxially extendingcylindrical recesses 110 are formed in a region adjacent to the recesses107 and provide guidance for the pushing member 120. Adjacent to therecesses 110, the front portions 105 a of the arms may include, at theirinner side, a widened diameter section 111 to provide sufficient spacefor permitting the pushing member 120 to slide therethrough. A steppedportion 111 a is formed between the recess 110 and the widened diametersection 111, which may form an abutment for a stepped portion at anouter surface of the pushing member, as explained further below.

Turning now in particular to FIGS. 18 to 20, the pushing member 120 willbe described. As generally shown in FIG. 1, the pushing member 120 maybe connected at a rear side thereof to a knob 121 which is adapted todisplace the pushing member 120 relative to the holding member 101. Inthe handle 102, there may be additionally housed a mechanism (not shown)that translates a rotating movement of the knob 121 into an axialdisplacement of the pushing member 120 relative to the holding member101. Various mechanisms may be contemplated to displace the pushingmember 120 relative to the holding member. For example, a mechanism maybe designed such that by rotating the knob 121, the pushing member canbe pushed axially to a maximum position relative to the holding member,and where further rotating the knob 121 will retract the pushing memberfrom the maximum axial position.

The front portion of the pushing member 120 is shown in detail in FIGS.18 to 20. Adjacent to a free end 120 a, the pushing member 120 includesa first substantially cylindrical section 122 with an outer diameterthat is configured to fit into the axial cylindrical recesses 110 at theinner wall of the front portions 105 a of the holding member (see alsoFIG. 13). Adjacent or close to the free end 120 a, an elongateengagement portion 123 is provided that extends transverse to thelongitudinal axis T, in particular, that extends perpendicular to thelongitudinal axis. The elongate engagement portion 123 has in theembodiment shown a rod-shape, and may be realized, for example, by a pinextending through a transverse hole in the front portion 122. A lengthof the elongate engagement portion 123 is at least greater than thedistance between the projections 44 on the locking ring 40, such thatthe rod support surface 44 a of the projections 44 can be engaged by theelongate engagement portion 123. In other words, the elongate engagementportion 123 forms a kind of dummy rod that is configured to push ontothe rod support surface 44 a. Adjacent to the first cylindrical section122, a second section 124 which has a slightly greater outer diametermay be provided so that a shoulder 122 a is formed between the secondsection 124 and the first section 122. The shoulder is configured toabut against the stepped portion 111 a in the inner side of the holdingmember 101. The length of the second section may be such that it extendsfurther proximally into the holding member than a length of the frontportions 105 a of the arms. Furthermore, for more stability, a thirdsection 125 with a still greater diameter may be provided.

The front portion 105 a of the arms is configured to be inserted intothe rod receiving portion 21 and is sized such that when the abutmentface 106 abuts against the upper end 2 a of the receiving part 20, thelegs 107 a extend to a short distance from the bottom 24 a of thesubstantially U-shaped recess 24 of the receiving part 20. In thisposition, the engagement portion 109 is at an axial height that permitsengagement with the engagement structure 32 at the receiving part.

The parts and portions of the bone anchor, the coupling device, and theinstrument may be made of any material, preferably, however, of abio-compatible material, such as titanium or stainless steel, or anyother bio-compatible metal or metal alloy, or a plastic material. As abio-compatible alloy, a NiTi alloy, for example, Nitinol, may be used.Other materials that can be used are magnesium or magnesium alloys.Bio-compatible plastic materials that can also be used may be, forexample, polyether ether ketone (PEEK) or poly-L-lactide acid (PLLA).The parts can be made of the same or of different materials from oneanother.

Use of the coupling device 10 and the instrument 100 will be describedwith reference to FIGS. 21a to 24c . First, steps of assembling thecoupling device 10 and the instrument 100 will be described. Thecoupling device may include the receiving part 20 and the locking ring40 in a pre-assembled manner. As shown in FIG. 21a , the pushing member120 is in a retracted position farther away from the free end portion101 a of the holding member than the engagement portions 109. Thelocking ring 40 is mounted onto the receiving part such that theprojections 44 engage the cutouts 33 of the receiving part. Forengagement with the instrument 100, the locking ring 40 may be at theinsertion position in which the edge 45 a of the arms 45 abuts againstthe axially higher located latching step 31 a. In the insertionposition, the head 3 can enter into the hollow interior 35 of the headreceiving portion 22. The distance between the front portions 105 a ofthe arms 105 is such that when the front portions of the arms areintroduced between the legs 25 of the coupling device, the frontportions 105 a are compressed towards each other.

As shown in FIG. 21b , when the front portions 105 a of the arms arecompressed towards each other by the inner wall of the legs 25, thefront portions 105 a can advance deeper into the passage 23. When theengagement portions 109 at the outer wall of the front portions 105 a ofthe arms reach the engagement structure 32 at the receiving part 20, theengagement portions 109 snap into the engagement structure 32 asdepicted in FIG. 21c . The holding member 101 can then be fixedlyconnected to the receiving part 20, since the front portions 105 a ofthe arms 105 can be prevented from being compressed inwards. As aresult, the coupling device 10 can be fixedly connected to theinstrument. FIG. 21d shows the fixed connection between the holdingmember 101 and the receiving part 20, with the pushing member 120 at apushed-forward position in which the elongate engagement portion 123contacts the rod support surface 44 a.

FIGS. 22 and 23 show in a more detailed perspective view the positionsof the pushing member 120 relative to the holding member 101. In FIG.22, the pushing member is at a position above the engagement portions109 and the engagement structure 32, and begins to spread the frontportions 105 a of the arms as it moves down. In FIG. 23, the pushingmember has been pushed downward until the elongate engagement portion123 is slightly above the rod support surface 44 a of the projections44. The elongate engagement portion 123 may not yet contact the rodsupport surface 44 a. From this position, only a small axial movement ofthe pushing member farther downwards is necessary for the elongateengagement portion 123 to push onto the rod support surface 44 a. Thisin turn moves the locking ring 40 downward, so that the annularprojection 41 of the locking ring can engage the bottom portion 37 a atthe outer surface of the head receiving portion 22. Thereby, the loweropening of the hollow interior 35 can be narrowed in such a manner thatan inserted head 3 cannot be removed.

Referring to FIGS. 24a to 24c , the placement and use of the couplingdevice 10 and the instrument 100 together with a bone anchor 1 areshown. In FIG. 24a the shank 2 has been inserted into a bone, forexample into a vertebra 500 prior to mounting the coupling device ontothe head. The instrument is fixedly connected to the coupling device andthe locking ring is at the insertion position wherein the head receivingportion can expand to let the head 3 enter. The engagement portion 123does not yet press onto the rod support surface 44 a. As depicted inFIG. 24b , the coupling device is placed by means of the instrument 100onto the head 3. In doing this, the head 3 snaps into the hollowinterior 35 of the head receiving portion 22 and may be held there byfriction. In a next step shown in FIG. 24c , the pushing member 120 ismoved towards or downward against the locking ring 40. Further downwardmovement of the pushing member pushes the locking ring 40 downward untilthe engagement portion 123 contacts the rod support surface 44 a of theprojections 44, as shown in FIG. 24c . Thereby, the arms 45 snap withtheir edge 45 a into the lower latching recess 31 b at the receivingpart. The locking ring 40 is now latched at the pre-locking position. Inaddition, the ledge 47 b of the projection 47 may snap under the step 34at the outer surface of the rod receiving portion which further securesthe pre-locking position. After achieving the pre-locking position, thepushing member 120 is retracted. When the pushing member 120 is moved toa higher position in which it no longer blocks inward movement of theengagement portions 109 and detachment from the engagement structure 32,the flexible front portions 105 a of the arms can be compressed towardseach other and the instrument can be removed from the coupling device.

After placement of the coupling device, the polyaxial bone anchoringdevice is in the pre-locking position. The head may be held by frictionin the seat so that the coupling device can assume an angular positionrelative to the shank which can be maintained provisionally prior tofinal locking.

In clinical use, a plurality of polyaxial bone anchoring devices areinserted into bone parts or into vertebrae, as shown in FIG. 25, forexample, into the pedicles of vertebrae 500. The coupling devices 10 arethen aligned so that a rod (not shown) can be received in the rodreceiving portions of two or more of the bone anchoring devices. With,for example, another instrument 300 different from the instrument 100described above, as shown schematically in FIG. 25, it may further bepossible to move the locking ring from the pre-locking position to alocking position and also to release the locking position. In thelocking position, the locking ring 40 is moved further downward untilthe head receiving portion 22 is firmly compressed so that the head islocked therein. The instrument 300 may include, for example, an outertube for engaging the locking ring, for example at the engagementportions 28, and an inner tube for engaging the rod receiving portion.The inner tube may be displaceable relative to the outer tube, so thatby displacing the outer tube relative to the inner tube, the lockingring can be moved from the pre-locking position to the locking positionand vice versa. The adjustments of the coupling device relative to thebone anchor can be performed without a rod and/or a locking memberplaced into the rod receiving portion, or the rod can be at an elevatedposition relative to the bottom 24 a of the recess 24. It shall be notedthat locking and unlocking can be carried out several times. When thecoupling device has been adjusted to a correct or desired position, thecoupling device can be locked to the bone anchor using a locking memberscrewed between the legs 25. Finally, the upper portion of the legs maybe broken-off.

Modifications of the embodiments described may further be contemplated.The parts are not limited to their detailed shape as depicted in theembodiments. In particular, the specific shape of the receiving part andof the locking ring may vary. The shape and structure of the engagementbetween the receiving part and the instrument can also be different fromthe embodiments shown. The design of the instrument for placing thecoupling device onto the head, in particular the front portion, is notlimited to the specific designs shown in the embodiments. Other shapesmay also be suitable. For example, the instrument may also engage thereceiving part at an outer surface thereof.

The rod can be any elongate device that is configured to connect twobone anchoring devices. For the bone anchor, all kinds of bone anchors,such as bone screws, bone nails, etc., may be used.

While the present invention has been described in connection withcertain exemplary embodiments, it is to be understood that the inventionis not limited to the disclosed embodiments, but is instead intended tocover various modifications and equivalent arrangements included withinthe spirit and scope of the appended claims, and equivalents thereof.

1. A coupling device for coupling a rod to a bone anchor, the couplingdevice comprising: a receiving part comprising: a head receiving portiondefining a seat for pivotably receiving a head of the bone anchor, thehead receiving portion being expandable to permit insertion of the headand compressible to lock the head in the seat; and a rod receivingportion defining a recess for receiving the rod, the recess having abottom and forming two legs, wherein the rod receiving portion comprisesan engagement structure for engaging a locking member to lock the rod inthe recess, and a first engagement surface different from the engagementstructure for engaging an instrument; and a locking ring positionablearound the head receiving portion, the locking ring comprising a secondengagement surface for engaging the instrument; wherein when the lockingring is around the head receiving portion, the locking ring can assumean insertion position where the head of the bone anchor is insertableinto the head receiving portion, and a pre-locking position where thehead is prevented from removal from the head receiving portion.
 2. Thecoupling device of claim 1, wherein the first engagement surfacecomprises a recess or a projection engageable with a correspondingprojection or recess at the instrument.
 3. The coupling device of claim2, wherein the recess or projection is formed on each of the legs. 4.(canceled)
 5. The coupling device of claim 1, wherein the secondengagement surface forms a rod support for an inserted rod.
 6. Thecoupling device of claim 5, wherein the rod support comprises twoprojections that are offset by 180° from one another.
 7. The couplingdevice of claim 1, wherein the receiving part comprises a thirdengagement surface engageable with the locking ring to restrict movementof the locking ring from the insertion position in a direction away fromthe pre-locking position.
 8. The coupling device of claim 1, wherein thereceiving part comprises a third engagement surface engageable with thelocking ring to restrict movement of the locking ring from thepre-locking position towards the insertion position.
 9. The couplingdevice of claim 1, wherein the locking ring can further assume a lockingposition relative to the head receiving portion where the head is lockedrelative to the receiving part.
 10. An instrument for placing a couplingdevice onto a head of a bone anchor, the coupling device comprising areceiving part comprising a head receiving portion for pivotablyreceiving the head of the bone anchor and a rod receiving portion forreceiving a rod, and a locking ring movable around the head receivingportion from an insertion position where the head of the bone anchor isinsertable into the head receiving portion to a pre-locking positionwhere the head is prevented from removal from the head receivingportion, the instrument having a longitudinal axis and comprising: aholding member having a front portion engageable with the receivingpart; and a pushing member that is movable axially relative to theholding member to engage the locking ring.
 11. The instrument of claim10, wherein the front portion of the holding member comprises two armsinsertable between legs of the receiving part to engage the rodreceiving portion.
 12. The instrument of claim 11, wherein the arms aremovable in the direction transverse to the longitudinal axis andcomprise engagement portions for engaging the legs. 13-14. (canceled)15. A system comprising the instrument of claim 10 and the couplingdevice comprising the receiving part and the locking ring.
 16. A methodof connecting a coupling device for a rod to a bone with a bone anchor,the coupling device comprising a receiving part comprising a headreceiving portion defining a seat for pivotably receiving a head of thebone anchor, the head receiving portion being expandable to permitinsertion of the head and compressible to lock the head in the seat, arod receiving portion defining a recess for receiving the rod, therecess having a bottom and forming two legs, wherein the rod receivingportion comprises an engagement structure for engaging a locking memberto lock the rod in the recess, and a first engagement surface differentfrom the engagement structure for engaging an instrument, and a lockingring positionable around the head receiving portion, the locking ringcomprising a second engagement surface for engaging the instrument, themethod comprising: anchoring the bone anchor to bone; inserting the headof the bone anchor into the head receiving portion of the couplingdevice when the locking ring is around the head receiving portion at aninsertion position; adjusting the locking ring from the insertionposition to a pre-locking position where the head is prevented fromremoval from the head receiving portion; and adjusting an angularposition of the coupling device relative to the bone anchor.
 17. Themethod of claim 16, wherein the bone anchor is anchored to bone prior toinserting the head of the bone anchor into the head receiving portion ofthe coupling device.
 18. The method of claim 16, further comprisingconnecting an instrument to the first engagement surface of thereceiving part to facilitate the insertion of the head of the boneanchor into the head receiving portion of the coupling device.
 19. Themethod of claim 18, wherein the instrument is further configured toengage the second engagement surface to adjust the position of thelocking ring.
 20. The method of claim 18, further comprising using theinstrument to adjust the locking ring from the insertion position to thepre-locking position to prevent removal of the head from the headreceiving portion.
 21. The method of claim 16, further comprisinginserting a rod into the recess after the adjusting of the angularposition of the coupling device relative to the bone anchor, andadvancing the locking member into the recess to lock the rod and thebone anchor relative to the coupling device.
 22. (canceled)